The invention relates to ceramic-metal compositions or cermets. More particularly the invention relates to densified cermets that have selected desirable characteristics that are derived from the individual ceramic and metal starting components.
The potential of ceramic-metal compositions that include a unique combination of properties, such as the hardness of a ceramic material combined with the ductility of a metal, has long been of interest. It has, however, proved difficult to achieve cermet compositions that are fully densified and have a particularly desired range of properties. The difficulties arise from the conflicting physical and chemical nature of the starting materials.
Major areas of difficulties in processing of ceramic-metal composites are associated with:
(i) chemical reactivity of the starting materials resulting in oxidation and/or metal depletion and formation of undesirable phases between ceramic and metal; and PA1 (ii) non-wetting behavior of the metal with respect to the ceramic component.
Chemical reactions between composite ceramic and metal components that occur before density is achieved are often undesirable, because metal required for densification is reacted to form variety of phases. These cermet products will likely have a higher porosity than desired and lack sufficient quantities of desired ceramic-metal phases that would, if present, impart needed properties to the cermet.
Reactions after density is achieved can be beneficial because desired quantities of ceramic phases can be controllably developed to impart such qualities as hardness and wear resistance to resulting products.
Where articles having a high ceramic content are desired, in order to take advantage of the high hardness and wear resistance of the ceramic component, densification becomes particularly difficult. For example, in a highly reactive system such as B.sub.4 C/Al, achieving a fully densified composition is difficult because there is significant reaction of Al with B.sub.4 C as the metal reaches a temperature where it wets the ceramic material and would otherwise completely fill the pores of or infiltrate into a porous ceramic body or preform.
Such densification difficulties led to an approach described by Pyzik, et al, in U.S. Pat. No. 4,702,770 wherein molten aluminum metal is infiltrated into a porous ceramic body of sintered B.sub.4 C, under vacuum conditions. The lower densification temperatures required for this process reduce unwanted Al-B.sub.4 C reactions.
Alternatively, B.sub.4 C-Al cermets may be obtained by infiltration of metal into chemically treated B.sub.4 C, such as described by Halverson et al., in U.S. Pat. No. 4,605,440 and U.S. Pat. No. 4,718,941. The advantage of Halverson's process is that the formation of a wide variety of ceramic B-C-Al phases having advantageous qualities is possible, although there is considerable difficulty in controlling the kinetics of chemical reactions and the final product character. A limitation of the Halverson process is that "packing efficiency" of the particulate ceramic in forming the porous ceramic body or green body necessary for infiltration is such that final product ceramic content is practically limited to B.sub.4 C contents of about 66 volume percent.
Pyzik et al., in the aforementioned U.S. Pat. No. 4,702,770, avoids the requirement of chemically pretreating ceramic materials and the limitation of low ceramic content of the cermet by sintering the porous ceramic body prior to metal infiltration. The sintering step allows ceramic contents of 66 to 95 percent of theoretical(100%) density. However, the sintering step, as required by the U.S. Pat. No. 4,702,770 process, profoundly reduces the reactivity of the ceramic with respect to the metal at the infiltration temperatures of 1150.degree.-1200.degree. C. necessary to assure wetting and achieve penetration of metal into the porous ceramic body. As a result, the only major phases remaining :n the densified cermet product are B.sub.4 C and Al. For higher density B.sub.4 C preforms, sintering temperatures above 2200.degree. C. are required and at such temperatures, the system becomes so chemically stable that even prolonged contact with aluminum does not result in significant reaction and formation of ceramic phases. While these B.sub.4 C-Al cermets do achieve higher ceramic contents, control of the chemical reactivity of the metal-ceramic systems, permitting formation of advantageous B-C-Al phases, is lost.
Thus, two general types of B.sub.4 C-Al cermets are known in the art heretofore. One type of B.sub.4 C-Al cermet is characterized by high reactivity for forming B-C-Al phases and low, less than 70 volume percent, ceramic content. The second type of B.sub.4 C-Al cermet is characterized by low reactivity for developing desired ceramic phases and high, greater than 70 percent, ceramic content.
In light of the deficiencies of the known B.sub.4 C-Al cermets, it would be desirable to produce ceramic-metal compositions of reactive ceramic metal systems that have substantially greater ceramic content than 70 volume percent, are substantially fully densified and retain sufficient reactivity of the ceramic with respect to the metal to allow the controlled formation of further advantageous ceramic phases by subsequent processing.